taru-adjective, adverb taking the 'to' particle
Literary, classical-style adverb/adjective describing something happening abruptly. Often used with と (忽焉として). Cross-referenced with 忽然 (こつぜん), which is more common in modern Japanese.
See also: 忽然
忽焉として風が止み、あたりは静寂に包まれた。
Suddenly the wind stopped, and the area was enveloped in silence.